Ink jet technology is increasingly being used for computer printers. Ink jet printing systems use the ejection of tiny droplets of ink to produce an image. The devices produce highly reproducible and controllable droplets, so that a droplet may be printed at a location specified by digitally stored image data. Most ink jet printing systems fall into one of two classifications. A "continuous jet" type ink jet printing system continuously ejects droplets from the printhead and directs the droplets either toward the paper or into a reservoir for recycling purposes. Continuous jet systems are based upon the phenomenon of uniform droplet formation from a stream of liquid issuing from an orifice. It has been previously observed that fluid ejected under pressure from an orifice about 50 to 80 microns in diameter tends to break up into uniform droplets upon the amplification of capillary waves induced onto the jet, for example, by an electromechanical device that causes pressure oscillations to propagate through the fluid. One drawback to continuous jet systems is that the fluid must be jetting even when little or no printing is required. This requirement degrades the ink and decreases reliability of the printing system.
A "drop-on-demand" type ink jet printing system ejects droplets from the printhead in response to a specific command related to the image to be produced. Typically, the ejection of a droplet is caused by an electromechanically induced pressure wave. In this type of system, a volumetric change in the fluid is induced by the application of voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to produce a droplet that issues from an orifice. Since the voltage is applied only when a droplet is desired, these types of ink jet printing systems are referred to as drop-on-demand.
The use of piezoelectric materials in ink jet printers is well known. Most commonly, piezoelectric material is used in a piezoelectric transducer by which electric energy is converted into mechanical energy by applying an electric field across the material, thereby causing the piezoelectric material to deform. This ability to distort piezoelectric material has often been utilized in order to force the ejection of ink from the ink-carrying channels of ink jet printers. One such ink jet printer configuration which utilizes the distortion of a piezoelectric material to eject ink includes a tubular piezoelectric transducer which surrounds an ink-carrying channel. When the transducer is excited by the application of an electrical voltage pulse, the ink-carrying channel is compressed and a drop of ink is ejected from the channel. For example, an ink jet printer which utilizes circular transducers may be seen by reference to U.S. Pat. No. 3,857,045 to Zoltan. However, the relatively complicated arrangement of the piezoelectric transducer and the associated ink-carrying channel causes such devices to be relatively time-consuming and expensive to manufacture.
In order to reduce the per ink-carrying channel (or "jet") manufacturing cost of an ink jet printhead, in particular, those ink jet printheads having a piezoelectric actuator, it has long been desired to produce an ink jet printhead having a channel array in which the individual channels which comprise the array are arranged such that the spacing between adjacent channels is relatively small. For example, it would be very desirable to construct an ink jet printhead having a channel array where adjacent channels are spaced between approximately three and six mils apart. Such an ink jet printhead is hereby defined as a "high density" ink jet printhead. In addition to a reduction in the per ink-carrying channel manufacturing cost, another advantage which would result from the manufacture of an ink jet printhead with a high channel density would be an increase in printer speed. However, the very close spacing between channels in the proposed high density ink jet printhead has long been a major problem in the manufacture of such printheads.
Many attempts to manufacture ink jet printheads having piezoelectric actuators and reduced spacing between channels have focussed on the manufacture of ink jet printheads with parallel channel arrays and shear mode piezoelectric transducers for actuating the channels. For example, U.S. Pat. Nos. 4,584,590 and 4,825,227, both to Fischbeck et al., disclose shear mode piezoelectric transducers for a parallel channel array ink jet printhead. In both of the Fischbeck et al. patents, a series of open ended parallel ink pressure chambers are covered with a sheet of a piezoelectric material along their roofs. Electrodes are provided on opposite sides of the sheet of piezoelectric material such that positive electrodes are positioned above the vertical walls separating pressure chambers and negative electrodes are positioned over the chamber itself. When an electric field is provided across the electrodes, the piezoelectric material, which is poled in a direction normal to the electric field direction, distorts in a shear mode configuration to compress the ink pressure chamber. In these configurations, however, much of the piezoelectric material is inactive. Furthermore, the extent of deformation of the piezoelectric interest is small.
An ink jet printhead having a parallel channel array and which utilizes piezoelectric materials to construct the sidewalls of the ink-carrying channels may be seen by reference to U.S. Pat. No. 4,536,097 to Nilsson. In Nilsson, an ink jet channel matrix is formed by a series of strips of a piezoelectric material disposed in spaced parallel relationships and covered on opposite sides by first and second plates. One plate is constructed of a conductive material and forms a shared electrode for all of the strips of piezoelectric material. On the other side of the strips, electrical contacts are used to electrically connect channel defining pairs of the strips of piezoelectric material. When a voltage is applied to the two strips of piezoelectric material which define a channel, the strips become narrower and higher such that the enclosed cross-sectional area of the channel is enlarged and ink is drawn into the channel. When the voltage is removed, the strips return to their original shape, thereby reducing channel volume and ejecting ink therefrom.
An ink jet printhead having a parallel ink-carrying channel array and which utilizes piezoelectric material to form a shear mode actuator for the vertical walls of the channel has also been disclosed. For example, U.S. Pat. Nos. 4,879,568 to Bartky et al. and 4,887,100 to Michaelis et al. each disclose an ink jet printhead channel array in which a piezoelectric material is used as the vertical wall along the entire length of each channel forming the array. In these configurations, the vertical channel walls are constructed of two oppositely poled pieces of piezoelectric material mounted next to each other and sandwiched between top and bottom walls to form the ink channels. Once the ink channels are formed, electrodes are then deposited along the entire height of the vertical channel wall. When an electric field normal to the poling direction of the pieces of piezoelectric material is generated between the electrodes, the vertical channel wall distorts to compress the ink jet channel in a shear mode fashion.
The manufacture of ink jet printheads having parallel channel arrays with sidewall actuators such as those disclosed by Bartky et al. and Michaelis et al. would be quite cumbersome in practice. To form such an ink jet printhead, a base wall would first be provided and a layer of piezoelectric material mounted thereon. A multiplicity of parallel grooves which extend through the piezoelectric material would then be formed, thereby providing the sidewalls which define the channels of the array. Electrodes would then be mounted on the surfaces of the sidewalls which define the channels so that the electric field required to displace the sidewalls may be applied. Electrical drive circuit means would then be connected and a top wall secured to the piezoelectric sidewalls to close the channels. In particular, mounting electrodes on the surfaces of the sidewalls which define the channels can prove quite difficult in practice, particularly in view of the very small dimensions typically involved. One method to mount electrodes along the surfaces of the sidewalls defining the channels would be to metallize the piezoelectric material along the surfaces, remove the metal from the tops of the walls forming the deep grooves and then making electrical connections to the walls deep within the grooves. It is anticipated that each of these steps would pose significant manufacturing problems.
One problem with manufacture of ink jet printheads using polarized piezoelectric material is the magnitude of the electric field necessary for polarization. Generally, at least 500 volts per mil is necessary for polarization. Consequently, for a long printhead, such as a page width printhead which must be at least 11 inches in length, an extremely high electric field would be necessary.
It would be most desirable to have a printhead which is as long as the widest paper size supported (i.e., a "page width" printhead). A page width printhead has several advantages. First, movement of the printhead may be eliminated or greatly reduced, thereby increasing the reliability of the system. Second, the speed of printing is significantly increased. Third, because the range of movement is limited or eliminated, the resultant image is of much greater quality.
Therefore, a need has arisen for a wide printhead using a high density parallel channel array in a polarized piezoelectric material.